Fluid balance in the lung is critical to survival. One million people are hospitalized each year for pneumonia, acute lung injury, or acute respiratory distress syndrome, diseases which compromise oxygenation from the failure of gas exchange in the lungs due to alveolar flooding. Alveolar fluid clearance is driven by active ion transport across the alveolar epithelium, composed of type I cells, which cover >95% of the internal surface area of the lung, and type II cells, which line ~5%. The generally accepted theory of ion and fluid transport in the lung is that type II cells, known to contain ion transport proteins, govern alveolar fluid balance by regulating Na+ transport, while type I cells, which contain water channels, merely provide a route for passive water absorption. However, the extensive alveolar surface area occupied by type I cells suggests that these cells may play a larger role in regulating lung fluid balance. Our preliminary data demonstrate that there are functional ion channels in type I cells, and that there are significant differences in how Na+ and Cl- transport are regulated in type I and type II cells. The underlying hypotheses for the studies in this application are that ion transport in type I and type II cells is regulated in part by different mechanisms and that the effects of agents that modulate alveolar fluid clearance in whole lungs are better explained by the effects of these agents on type I, rather than type II, cells. Specific Aim 1 will determine regulatory mechanisms of Na+ transport in type I cells by studying modulation of ENaC and Na+-, K+-ATPase. Specific Aim 2 will explore mechanisms of amiloride-insensitive Na+ transport in type I cells. Specific Aim 3 willstudy anion transport in type I cells and determine the roles of adenosine and beta-agonists on Cl- flux regulation. Elucidating the mechanisms of lung fluid homeostasis will be invaluable in developing strategies to treat alveolar flooding, as such therapies do not currently exist. The UCSF Department of Medicine and the Cardiovascular Research Institute provide ideal settings for training physician-scientists, combining multiple resources and a scientific community that is committed and excited about training future academic researchers. This proposal will develop expertise in ion transport in alveolar type I cells with the ultimate objective of clarifying how the lungs regulate lung fluid balance. The research and career development plan will aid in the candidate's goal to become an independent investigator with an academic career in Pulmonary Medicine.